The present invention relates to rotary regenerative air preheaters for the transfer of heat from a flue gas stream to a combustion air stream. More particularly, the present invention relates to a quad sector air preheater.
Rotary regenerative air preheaters are commonly used to transfer heat from the flue gases exiting a furnace to the incoming combustion air. Conventional rotary regenerative air preheaters have a rotor supporting heat transfer surfaces. The rotor is rotatably mounted in a housing. The rotor has radial partitions or diaphragms defining compartments therebetween for supporting the heat transfer elements. Sector plates extend across the upper and lower faces of the rotor to divide the preheater into a gas sector and an air sector. The hot flue gas stream is directed through the gas sector of the preheater and transfers heat to the heat transfer surfaces on the continuously rotating rotor. The heat transfer surfaces are then rotated to the air sector of the preheater. The combustion air directed over the heat transfer surfaces is thereby heated.
Large steam generators utilizing pulverized coal firing, typically employ a portion of the heated incoming combustion air for drying classification and transport of the coal in the pulverizer. It is normally required that coal be dried and pulverized before ignition can take place. The portion of the incoming air directed to the pulverizer is referred to as primary air. The remaining heated combustion air, referred to as secondary air, is directly sent to the steam generator.
In one prior embodiment, the primary air for the pulverizer is directed through a first preheater, and the secondary air for direct use in combustion is directed through a second preheater. Conventionally, to eliminate the costly and complex requirement of multiple preheaters, a single preheater having multiple air sectors has been used to eliminate the requirement for multiple air preheaters. These tri-sector preheaters have the air sector of the preheater subdivided by an additional air sector plate.
Radial sealing members along the edges of the partitions or diaphragms of the rotor wipe against the air-gas sector plates which divide the preheater into the flue gas sector and the air sector. The radial sealing members further wipe against the sector plate which divides the air sector of the preheater into the primary and secondary air sectors, sometimes referred to as the priair-secair sector plates. The sealing engagement of these radial seals with the sector plates minimizes the leakage and mixing of the flue gas stream with the air stream, and the primary air stream and secondary air stream. In order to keep the leakage as low as practicable, it is common to provide a double sealing arrangement between the rotor and the sector plates. In this arrangement, the air-gas sector plates and primary air-secondary air sector plate are equal in size to two rotor compartments. With this arrangement the radial seals on two consecutive partitions or diaphragms are in engagement with the sector plate at the same time.
A deficiency of conventional tri-sector preheaters is that the sector plates required for the double seal arrangement occupy or block off a non-trivial percentage of the flow area through the air preheater. This blockage reduces the flow area through the rotor and increases the pressure drop through the preheater requiring that the size of the preheater be increased to compensate.
The double sealing arrangement of the radial seals with the sector plates, along with axial seals arranged along the outer circumference of the rotor and the housing, prevent leakage of air and flue gas among the flue gas, primary air and secondary air sectors. Typically, the most significant air leakage among the sectors is direct leakage. Direct leakage is the quantity of air that passes between the radial and axial seals and sealing surfaces as a result of pressure differentials between the different air and flue gas streams. Conventionally, the primary air stream operates at the highest relative pressure in order to adequately dry and convey coal from the pulverizers. The secondary air stream is conventionally at a pressure greater than the ambient air pressure, but less than the pressure of the primary air steam. The flue gas stream is typically at a pressure below the ambient air pressure due to the downstream positioning of fans for the movement of the flue gas. Therefore, direct leakage typically occurs between the primary air sector and the flue gas sector and between the secondary air and flue gas sector, due to the differential pressures between the sectors.
Direct leakage can dilute or decrease the temperature of the flue gas by 10.degree. F. to 20.degree. F. The cooler combustion air stream can mix, through direct leakage, with the hotter flue gas stream to reduce the flue gas stream exit temperature. Reduced flue gas stream exit temperatures lower the cold end metal temperatures of the rotor. The cold end of the rotor can therefore fall below the dewpoint of the flue gas. Consequently, steel construction materials in the rotor are subject to corrosion from sulfuric acid as moisture condenses on the rotor in the presence of sulfur in the gas. As the percentage of sulfur in the coal rises, the amount of potential corrosion to the cold end increases. The corrosion can lead to more frequent replacement of corroded cold end components. Furthermore, for coal firing, fouling potential increases as the temperature decreases.
In addition, direct leakage between the air sector and the flue gas sector reduces air side flow. Therefore larger fans are required increasing initial and operating expenses.